The present invention relates in general to a process for fabricating a compact bipolar transistor which eliminates the need for an isolation field oxide region between the emitter and collector of the transistor.
Silicon bipolar transistors are often fabricated using a process in which separate portions of a single polycrystalline silicon layer form the contacts to the transistor's collector, base and emitter. To increase the switching speed of the transistor, the emitter must be made as small as possible to reduce parasitic capacitances in the transistor. Photolithographic techniques place inherent lower limits on the device dimensions that can be obtained. Non-photolithographic techniques have therefore been sought to reduce the device dimensions.
An example of one such technique is illustrated in U.S. Pat. No. 4,722,908 to Burton which discloses a bipolar transistor fabrication process in which polysilicon sidewalls are formed that serve both as dopant sources for the formation of underlying base and emitter regions, and as contacts for those devices. The thickness of the polysilicon sidewalls is precisely controllable through conventional polysilicon deposition techniques and determines the widths of the underlying device regions. Device widths of approximately 0.05 micron are obtainable using this process which is a very substantial improvement over the approximate 1 micron limit obtainable using conventional photolithographic techniques.
The process set forth in the Burton patent has a number of drawbacks. First, the design requires that an additional isolation field oxide region be formed between the collector contact region and the emitter of the transistor to prevent a short circuit between the two. This results in a structure that is not particularly compact. Second, the same polysilicon sidewall technique that is used to form the narrow emitter is also used to form the base contact. As a result, the base contact is also narrow which increases its resistance. Thus, there is a trade off between the increased operation speed realized from the narrow emitter and the undesirable increase in resistance of the base contact.